Compositions for the method of selectively dissolving nickel from other metals



United States Patent US. Cl. 75-91 Claims 10 ABSTRACT OF THE DISCLOSURE A nickel stripping composition comprising a polyphosphate, a nitro-organic compound and ammonia, modified by the inclusion of thiosulfate. The nickel stripping rate is catalyzed by the thiosulfate, and the composition may be made more or less selective in stripping nickel in the presence of copper by varying the concentration of the thiosulfate.

This invention is directed to chemical dissolution of a metal, and more particularly to chemically stripping nickel from substrates on which the nickel has been plated or otherwise applied.

A variety of articles in the hardware, automotive and appliance industries, for example, are commonly given a 5 plate or coating of nickel, sometimes alone but generally in conjunction with other metals to provide a protective and decorative finish. For one reason or another almost invariably there are some articles on which the nickel deposit is defective rendering those articles unacceptable commercially. Nevertheless there is frequently a substantial economic value in the articles if they can be salvaged by removing the defective plate. Accordingly there is extensive need in such industrial operations for an inexpensive and easily controlled method of removing the nickel plate.

There have been many system devised and used for stripping nickel plate from articles, including the use of cyanide compounds, mineral acids or strong caustics, as well as a number of othersystems which are not subject to the health hazards of the cyanide compounds or the highly corrosive action on the substrate of the acid or caustic systems. It is a main purpose of the present invention to provide an improved stripping solution of the noncyanide, selective type which affords substantially faster nickel stripping rates at lower operating temperatures and greater ease of stripping operation. It is a further object of the invention to provide a chemical stripping system for nickel plated articles which is stable and not adversely affected by incidental presence of other metal ions in the stripping bath which normally poison or interfere with the stripping of the nickel.

A further purpose of the novel stripping compositions is to provide a system which is selective in its action so that nickel may be stripped from a substrate without adversely affecting a subordinate metal, such as a deposit of copper, or, alternatively if desired, to strip both the nickel and such subordinate metal layer.

Among the various systems heretofore proposed for selective stripping of nickel from substrates are those involving the use of nitro organic compounds and ammonia or ammonium salts, amines, and in some cases a sulfur compound to provide sulfide ions in the solution. Such systems are disclosed more particularly in US. Patents Nos. 2,937,940; 3,102,808 and 3,163,524.

3,460,938 Patented Aug. 12, 1969 It has now been found that substantially improved nickel-copper stripping operation can be achieved by means of a system employing a nitro-substituted organic compound, a base such as ammonia or an amine, in conjunction with a water soluble polyphosphate as the main complexer for nickel and another complexer comprising a thiosulfate compound which mainly acts as a chelating agent for monovalent copper. The thiosulfate compound also has a catalytic efiect in the system in that it exerts an activating effect on the nickel, increasing the rate of dissolution over that obtainable in the absence of the thiosulfate, but it is included primarily for the different purpose explained hereinabove.

Polyphosphates in general and pyrophosphates in particular are known to form stable chelates with heavy metals. Use of polyphosphates in a metal stripping solution, however, is hampered by an extreme sensitivity to poisoning by copper and possibly also zinc ions which are slowly and unavoidably introduced into the stripping bath in the normal practical operation. This difficulty is overcome in stripping compositions according to themvention by the action of the thiosulfate which acts as a secondary complexer, eliminating the copper and possibly other metal ions in solution which interfere with the dissolution of the nickel.

It has been found that the nickel stripping rate of the new system is proportional to the concentration of the pyrophosphate at a given temperature, and that presence of the thiosulfate has no adverse efiect upon the nickel dissolution rate, in fact as already mentioned it tends to enhance the rate as well as to tie up any copper and prevents poisoning of the system. Moreover the system can be modified to strip copperas well as nickel, still without adverse elfect upon the nickel stripping capability, by increasing the concentration of the thiosulfate in solution.

Thus it is possible selectively to strip nickel from a copper or copper-clad substrate or to strip both copper and nickel at selected rates by modifying the relative concentrations of the pyrophosphate and the thiosulfate.

The invention is illustrated by the following example.

Example I A nickel stripping solution was prepared having the following composition:

Mol/l. Sodium acid pyrophosphat 0.5 Sodium m-nitrobenzene sulfonate 0.25

Sodium thiosulfate 0.5 Ammonium hydroxide to give a solution pH of 8 to 10;

approximately 1.0 mol/l.

The ammonia in this system is of the stoichiometric amount so as to be all combined with the 0.5 moL/l. of sodium acid pyrophosphate, leaving no free ammonia. This accounts for the stability of the ammonia in this system as compared to systems in the prior art where the ammonia present is in the free state, and thus more volatile.

A nickel plated steel article having a copper flash deposit underlying the nickel was immersed in the foregoing solution at ambient room temperature (77 F.) without agitation. The nickel plate was dissolved at the rate of 0.006 mil per minute. This rate stays substantially constant up to a nickel concentration in the stripping solution of at least 3 to 4 ounces per gallon.

The stripping rate is materially increased by raising the temperature of the solution. At F., the foregoing solution provides a stripping rate of 0.016 mil per minute of nickel. At temperatures intermediate these limits, the rate varies as a straight line function.

Example II Moi/l. Sodium m-nitrobenzene sulfonate 0.25 Ammonium sulfate 1.0 Ammonium hydroxide (to pH 9) 0.50

Example III The solution given in Example I above has minimal copper stripping action since the amount of thiosulfate present there is intended only to complex copper ions which incidentally and unavoidably get into the stripping bath solution owing to the presence, for example, of the copper strike on the articles being stripped. If, however, it is desired to strip copper as well as nickel in the bath, this can be readily done by increasing the concentration of the thiosulfate in the bath. The following is an example:

Mol/l. Sodium acid pyrophosphate 0.5 Sodium m-nitrobenzene sulfonate 0.25 Sodium thiosulfate 1.5

Ammonium hydroxide to give a pH of 8 to 10-approximately 1.0 mol/l.

The low solubility of sodium thiosulfate makes it necessary to operate this system at about 120 F. At such temperature, nickel is stripped at the rate of about 0.011 mil per minute, while copper is removed at the rate of about 0.019 mil per minute. This particular system produces a maximum rate of copper dissolution and it will be noted that there is no adverse effect on the stripping of the nickel.

Example IV Elimination of thiosulfate from the system has a dual effect. First of all, the dissolution rate of nickel is reduced. Thus in a solution the same as that of Example I except for omission of thiosulfate, the nickel stripping rate at room temperature is .0012 mil per minute. Moreover, such rates are obtained only in the absence of any copper ions in solution. When these appear, the system is essentially inoperative to dissolve nickel.

The effect on stripping rates of different concentrations of pyrophosphate and thiosulfate is illustrated by the values given in the following table.

TABLE 1 Stripping rate Cncentration(mo1sliiter) (mil/min. at 160 F.)

Sodium m-nitrobenzene Sodium acid Sodium suifonate pyrophosphate thiosuliate Nickel Copper The following observations can be made from Table 1. The nickel stripping rates, at constant pyrophosphate concentration, are independent of the thiosulfate concentration. The copper stripping rates, at constant pyrophosphate concentration, are dependent on thiosulfate concentration. The nickel stripping rate, at constant thiosulfate concentration, are dependent on pyrophosphate concentration.

And the copper stripping rates, at constant thiosulfate concentrations, are independent of pyrophosphate concentration.

Other polyphosphates than sodium acid pyrophosphate are useful in the practice of the invention. Such others include ammonium-sodium pyrophosphate which is formed in situ in Example I, as well as the tetrasodium phosphate. The equivalent potassium and lithium salts may likewise be employed although they are not preferred because of greater expense.

In a similar manner, either sodium tripolyphosphate or sodium hexametaphosphate may be employed in place of the sodium acid pyrophosphate in Example I. This is illustrated by the following:

Example V Moi/l. Sodium hexametaphosphate 0.5 Sodium thiosulfate 0.5 Sodium m-nitrobenzene sulfonate 0.25 Ammonium hydroxide 1.0

Using this solution, nickel is stripped from a steel base article (having a copper flash beneath the nickel) at the rate of .0033 mil per minute at a temperature of 75 F.

Example VI Moi/l. Sodium tripolyphosphate 0.5 Sodium thiosulfate 0.5 Sodium m-nitrobenzene sulfonate 0.25 Ammonium hydroxide 1.0

Using this solution, nickel is stripped from a steel base article (having a copper flash beneath the nickel) at the rate of .0021 mil per minute at a temperature of 75 F.

The nitro-substituted organic oxidizers that may be employed are well known and any of those used in the prior systems mentioned above are operative. In addition to the nitrobenzene sulfonates, they include nitrobenzoic acids, nitrophenols, nitroanilines, and mixtures of these, all being characterized by the fact that they are nitro-substituted aromatic compounds containing at least one nitro group attached to a benzene ring which ring also carries a radical having a solubilizing elfect on the nitro-substituted aromatic compound. The several ortho, meta and para forms of these compounds are all utilizable.

Example VI'I Stripping solutions were prepared as in Example I except that various nitro-substituted organic oxidizers replaced the sodium m-nitrobenzene sulfonate. The bath was operated at room temperature. The stripping rates of nickel from articles of the same type as that referred to in Example I are given in the following table:

' TABLE 2 Cone. Substitute oxidizer (moi/l) Sodium o'nitrobenzene suifonate Sodium o-nitrobenzoate Sodium m-nitrobenzoate... Sodium p-nitrobenzoate Sodium nitrophthalate Strl ping rate gnu/min.)

NNNIOIO a manac! Other oxidizing agents include nitro-substituted aliphatic compounds having at least some solubility in water Example VIII A solution the same as that in Example I was prepared except that nitropropane was substituted for the nitrobenzene sulfonatc of the prior example. Nitropropane was added in excess of its solubility and the stripping rate of nickel from an article of the same type as that used in Example I was .0010 mil per minute at 75 F. Here is was'necessary to use air agitation to disperse the slightlysoluble nitropropane. I

In place oLamntogipm hydroxide, ethylenediamine may be employed in a bath whose composifi'onis'otlierwise the same as in Example I. The same molar concentration of the base is maintained. At 75 F., the rate of nickel stripping is 0.0012 mil per minute.

The useful thiosulfates include not only the sodium compound but the other reasonably soluble alkali metal salts such as potassium and lithium, as well as the ammonium salt.

The system of the present invention is characterized by remarkable stability. -It has a distinct advantage overprevious systems in which the principal complexing agent for the nickel is ammonia. The low concentration of free ammonia in the invention systems eliminates the disadvantages of high loses on account of volatility of ammonia inherent in the prior systems utilizing this as the principal complexer. In contrast to the other systems, ammonia in the invention systems operates onlyas a secondary complexenand is apparently regenerated in the solution itself. Thus it appears that the nickel ions brought into solution arenot in the form of the well known, (Ni(NI-I complex, but are somehow combined with the P radical, possibly as a mixed It is also surprising that extremely long life is obtained in the stripping baths when it is considered that sodium thiosulfate is normally a strong reducing agent and is present in the invention solutions with a strong oxidizer. The combination appears incompatible and hence unstable. This is not the case, however, apparently because the reducing properties of thiosulfate are predominant mainly in acid solution and are suppressed in the alkaline medium of the invention provided by the presence of either ammonium hydroxide or ethylenediamine. It is further surprising to find that in spite of the normal reducing character of thiosulfate, it is useful in a stripping operation which isof course an oxidizing operation.

The combination of organic oxidizer, polyphosphate "primary complexer, thiosulfate secondary complexer and alkaline medium is thus unique in providing a nickel stripping bath which is noncorrosive to steel, has high stripping rate capability at ambient room temperatures, and maintains such rate substantially constant until depletion of the oxidizer and primary complexer occurs.

The concentrations of the oxidizer, polyphosphate and thiosulfate are not critical in effecting dissolution of nickel, although of course the rate of dissolution will vary with the concentration. For most practical applications, a stripping capacity of about 4 ounces of nickel per gallon of stripping solution is considered desirable. Thus the polyphosphate concentration of about 0.5 mol/l. in combination with about 0.25 mol/l. of nitro-substituted aromatic to achieve stoichiometric balance is generally selected. Apart from economics, the limiting concentraion-is determined by the solubility 'of'the "several com- --ponents. As previously "indicated, maximum copper" strip ping effect of the thiosulfate occurs at about 1.5 mols/l. but if this concentration is used, the solution temperature must be raised to about 120 to accept this amount of thiosulfate. Thus where room temperature operation is used, the thiosulfate concentration is reduced to about 0.5 mol/l. This is about the limit of its solubility in solution at that temperature. The concentrations of the oxidizer and polyphosphate, however, are still preferably maintained at 'the above-indicated levels. At lower concentrations the rate, as well of course as the nickel dissolution capacity, is reduced.

The solutioiis of the invention are operative over a rather broad range of pH from about 8 to 11. In a solution such as that of Example I when equivalent ammonium hydroxide is added to neutralize the acid pyrophosphate; the solution has a pH of 8.5." Thus it is apparent that the system is operative where the concentration of ammonium hydroxide is less than as well as more than the exact equivalent. The use of solutions at the lower end of the range will be limited by a corrosive tendancy toward steel, while solutions of higher pH will result in free ammonia which will give rise to volatalization losses.

As an indication of the exceptionally long life of the stripping compositions of the invention, a bath such as that illustrated in Example I above will be commercially operative even on an intermittent basis for at least several weeks, assuming of course the oxidant-complexer combination has not been exhausted by the dissolution of nickel. Throughout such period, the stripping rate remains virtually unchanged.

What is clair'ried is:

1. A composition which is aqueous solution is useful for selectively dissolving a nickel deposit from a substrate in which copper may also be present, said composition consisting essentially of:

(a) a water soluble polyphosphate of the group consisting of the sodium, potassium, ammonium and lithium salts and mixtures thereof;

(b) an organic oxidizer of the group consisting of nitrosubstituted'aromatic compounds containing at least one nitro group attached to a benzen ring ring which ring also carries a radical having a water-solubilizing effect on the nitro-substituted aromatic compound, and water soluble nitro-substituted aliphatic compounds having from 1 to 3 carbons in the chain;

(c) a base the group consisting of ammonia and ethylenenediamine; and

(d) a thiosulfate compound of the group consisting of the sodium potassium, ammonium, lithium salts and mixtures thereof;

said polyphosphate and oxidizer being present in substantially stoichiometric amounts up to the limiting solubility of the polyphosphate in order to complex the nickel as stripped, and said base being present in amount sufficient to maintain a pH of 8 to 11 in solution; said thiosulfate being present in small effective amount up to the limit of its solubility.'

2. A composition as defined in claim 1, wherein said polyphosphate isa pyrophosphate.

3. A composition as defined in claim 1, wherein said polyphosphate istn acid pyrophosphate.

4. A composition as defined in claim 1, which consists essentially, of an aqueous solution of sodium acid pyrophosphate, sodium nitrobenzene sulfonate, sodium thiosulfate and ammonium hydroxide.

5. A composition as defined in claim 4, wherein the sodium acid pyrophosphate is about 0.5 mol per liter, the sodium nitrobenzene sulfonate is about 0.25 mol per liter; the sodium thiosulfate is about 0.5 to 1.5 mols per liter, and the amount of ammonium hydroxide is about 1.0 mol per liter.

6. The method of stripping a nickel deposit from a substrate article in which copper may also be present which comprises immersing the article in an aqueous solution consisting essentially of (a) a water soluble polyphosphate compound of the group consisting of the sodium, potassium, ammonium, lithium salts and mixtures thereof;

(b) an organic oxidizing agent of the group consisting of the nitro-substituted aromatic compounds containing at least one nitro group attached to a benzene ring which ring also carries a radical having a water solubilizing effect on the nitro-substituted aromatic compound, and water-soluble nitro-substituted aliphatic compounds having from 1 to 3 carbons;

(c) a base of the group consisting of ammonia and ethylenediamine;

(d) a thiosulfate compound of the group consisting of the sodium, potassium, ammonium, lithium salts of said compound and mixtures thereof;

said polyphosphate and oxidizer being present in substantially stoichiometric amounts up to the limiting solubility of the polyphosphate in order to complex: the nickel as stripped, and said base being present in amount to produce a pH of 8 to 11 in solution; said thiosulfate being present in small but effective amount up to the limit of solubility; maintaining said solution between ambient room temperature and about 160 F. and keeping said articles immersed in said solution for a time sutficient to remove the nicltel therefrom and then withdrawing the articles from said solution.

7. A method as defined in claim 6, wherein the thiosulfate concentration is between 0.1 and 1.5 mol per liter.

8. A method as defined in claim 6, wherein said solution is maintained at room temperature and the thiosulfate concentration is about 0.5 mol per liter.

9. A method as defined in claim 6, wherein the thiosulfate concentration is 1.5 and the solution is maintained at a temperature of at least about 120 F.

10. A method as defined in claim 6, wherein said solution consists essentially apart from water of about 0.5 mol per liter of sodium acid pyrophosphate, about 0.25 mol per liter of sodium nitrobenzene sulfonate, about 0.5 mol per liter of sodium thiosulfate, and about 1.0 mol per liter of ammonium hydroxide to maintain a pH of from 8 to 11.

References Cited UNITED STATES PATENTS 3,102,808 9/1963 Weisberg et al. 75'97 3,163,524 12/ 1964 Weisberg et al. 75-103 3,203,787 8/1965 Grunwald 7597 3,351,556 11/1967 Tsourmas 75-97 X OSCAR R. VERTIZ, Primary Examiner G. O. PETERS, Assistant Examiner US. Cl. X.R. 75-119; 252-102 

